Previous studies have shown that the development of global form perception in humans is gradual when compared to other basic visual functions (e.g. visual acuity). The development of basic visual functions is mirrored in human and non-human primates. Few investigations of the development of object recognition and global form perception have been conducted in primate infants. A few studies of human infants suggest that global form perception is present near birth, while data from children suggest that it develops around age 5 years or later. Most studies of children confound development of language and perception, or specifically instruct the participants. To avoid these confounds, we used an animal model and Kanizsa illusory contours as an assay of global form perception. We recently showed that 1.5 year-old macaque monkeys demonstrate the ability to perceive Kanizsa illusory forms (Feltner & Kiorpes, VSS, 2008). We have now studied the time period during which this ability develops.

Using a modified 2-alternative forced-choice preferential looking paradigm, one infant pigtailed macaque monkey was tested at 6 months and 12 months of age. At 6 months, the monkey was able to differentiate novel from familiar real forms. The monkey was then tested with five Kanizsa illusory forms that were paired into four test conditions. Following a 2-sec habituation period, he was asked to correctly identify a novel illusory contour. The monkey showed chance performance averaged across the four test conditions (66%). However, at 12 months of age, he successfully demonstrated the ability to discriminate Kanizsa illusory contours (83%). These results indicate that the ability to discriminate differences between Kanizsa illusory forms develops between 6 and 12 months of age. Further, this evidence supports the idea that global form perception develops post-natally, at a more gradual rate than more basic visual functions.